Macrostemum -- the "Uncommon" Common Netspinner: Back to the North Fork

It's the strangest common netspinner we see in our streams, and the only place I've ever seen them in large numbers is the North Fork of the Rivanna below Advance Mills.  There's obviously something unique in the habitat there: I often find 6-8 per rock.

If you've not seen them, they'll shock you -- they're big.  They're long and they're fat, and I've been with stream workers who don't have a clue what they are.  Still, if you flip them over and take a look at that belly, there can't be much doubt about the ID.

Common netspinner (Hydropsychidae), genus Macrostemum.  (I'll turn to the problem of species ID in a minute.)  Macrostemum larvae are unique in a number of ways, in addition to being chubby and long (10 -- 22 mm) and an odd color of green.  1) They have a "carinate, flattened head." (Steven Beaty, "The Trichoptera of North Carolina," p. 77)  I.e. there's a U-shaped ridge that goes around the top of the head, and the top of the head is flat -- in fact it's almost "depressed" or "concave."

And 2) there is a "dense fringe of setae on [the] foretibiae and tarsi."  (Beaty, p. 77)

Both features also show up in this microscope photo that I took last year.

The genus Macrostemum is commonly known as the "Zebra caddis," no doubt because the adult is striped like a Zebra, with lines of tan and black.  I was hoping to find one today for this entry, but that didn't work out.  You might check out Tom Murray's photos on Bugguide.net: http://bugguide.net/node/view/13140/bgimage?from=24.  Thomas Ames also has a very nice photo in his book on Caddisflies, p. 147.

And now, for the problem of species ID.  There are only three Macrostemum species in North America, one of which -- M. transversum -- is rare.  So realistically, our choices are M. carolina or M. zebratum.  Both have a reddish brown head, and on both the back of the carina is "even" -- i.e. there is no bump or tubercle there.    So far, our larva could be either one.

M. carolina and M. zebratum differ in only one way -- the size of the tubercles near the eyes.  Are they large (= M. carolina), or small (=M. zebratum)?  Here are two photos of the tubercle at the top of the eye.
(Click to enlarge.)

Is it large or small?  Obviously we have no way to tell.  Large and small are relative terms that require a point of reference, something we don't have.

There is one other approach to species ID.  M. zebratum larvae are supposed to be 22 mm long when they're mature: M. carolina are presumably smaller since both Beaty (p. 77) and Ames (p. 148) claim that M. carolina adults are relatively small -- Ames says "The species [as an adult] "doesn't get much bigger than 14 mm."  The largest larva I collected today was 16mm, but I don't know if it was fully mature.  Will it continue to grow?  Will it reach 20-22 mm?

Best to leave this alone -- which, in the end, is what Beaty advises!  More photos below.